Stratigraphy of Fe/Mg Clays and Sulfates in Meridiani Planum region: possible implications for Mars' climatic transition.

Introduction: it is generally acknowledged that Mars past surface conditions were very different from what we observe today. The presence of fluvio-lacustrine morphological features and a widespread hydrous mineralogy on older surfaces indicate that water-rich conditions were common during the first billion years of the history of Mars 1. In particular, the presence of Fe/Mg phyllosilicates on Noachian surfaces (4.1-3.7 b.y. old) indicates that aqueous alteration of the crust at that time was happening at circumneutral pH conditions 2.The nature of Mars climate started to change around 3.7 b.y. ago, at the beginning of the Hesperian period (3.7-3.0 Ga), where we have indications that the environment had become drier and acidic. Evaporites, primarily sulfate-rich salts, are the major alteration mineralogy observed on Hesperian surfaces 1 and the occurrence of water-related morphologies decreased abruptly 3. Most of the local and regional records of the stratigraphic variability induced by this major climate change event are still unclear and poorly constrained but offer valuable information for assessing the possibility for lifes origin and endurance on Mars.Objective: we focus our investigations on the equatorial region of Mars called Meridiani Planum. This area is well-known for showing signs of a rich and varied aqueous activity spanning through the Noachian and the Hesperian. In particular, a thick sequence of layered sediments rich in sulfates and clays 4 is observed, potentially retaining key information on the climate and environment in which they deposited.Datasets and methods: We select several areas within the northern part of Meridiani Planum which show presence of layered sediments rich in hydrous minerals. Mineralogy and stratigraphy are investigated combining spectral information from the CRISM instrument with high resolution images and DEMs from CTX and HiRISE.Results and discussion: Stratigraphic analysis has evidenced that sulfates (polyhydrated and monohydrated Mg sulfates) are commonly observed at the bottom of the stratigraphic sequence, while clays (Fe/Mg phyllosilicates) are deposited on top, with no evidence of tectonic-structural phenomena that could have overturned the original stratigraphic sequence. The clays observed here therefore do not belong to the Noachian units of Meridiani planum but were formed later, after the sulfates were deposited. The transition between an environment which favors abundant sulfate deposition to one that favors Fe/Mg clays is recording a climatic transition which does not follow the general clay-to-sulfate trend observed at large scales on Mars 1, implying an additional level of complexity to the geologic history of Meridiani. It is essential to assess if similar stratigraphic sequences are observed elsewhere on Mars, to define whether this phenomenon is confined to this region or is evidence of larger scale, if not global, events.Acknowledgements: This project is partially funded by Europlanet RI20-24 GMAP project (research grant agreement No. 871149).References:1 J. Bibring et al. (2006), Science, 312. 2 S. L. Murchie et al. (2009), JGR-Planets, 114 (E2). 3 B. Hynek, et al. (2010),JGR-Planets, 115 (E9). 4 J. Flahaut et al. (2015), Icarus, 248, 269-288.